1. Technical Field
The invention relates to an objective lens capable of efficiently converging used light onto an optical recording medium when information is recorded or reproduced, an optical pickup device and a recording and/or reproducing apparatus for the optical recoding medium. Specifically, the invention relates to an objective lens for used in recording/reproducing a high density optical recording medium with blue light having a short wavelength, an optical pickup device, and a recording and/or reproducing apparatus for the optical recording medium.
2. Description of the Related Art
Recently, various optical recording media such as DVD (digital versatile disc) and CD (compact disc which includes CD-ROM, CD-R, and CD-RW) have been widely used in various applications. However, in response to a rapid increase in data volume, an increase in storage capacity of an optical recording medium has been strongly demanded. It has been known that a decrease in wavelength of used light of a light source and an increase in numerical aperture (NA) of an objective lens are effective to increase a storage capacity of an optical recording medium. Based on this knowledge, the blu-ray disc (herein after referred to as “BD”) having about 25 GB in a single sided single layer has been put to practical use. For the BD, light which is emitted from a semiconductor laser (for example, emitting laser light having a wavelength of 405 nm) having a short wavelength light output is used as irradiation light, and a numerical aperture is increased to be equal to or greater than 0.7. In the specifications of the BD, the numerical aperture and a thickness of a protection layer (for example, the numerical aperture (NA) is 0.85, and the thickness of the protection layer is 0.1 mm) are set quietly different from those of DVD and CD.
However, in future, a further increase in density will inevitably be demanded, but it might be hard to satisfy this demand by promoting a decrease in wavelength. This is because optical transmittance of lens materials is rapidly reduced in the range of a wavelength λ less than 350 nm and thus, it is hard to obtain sufficient optical efficiency in practice.
Another way for achieving high density is to further increase the numerical aperture of the objective lens.
Meanwhile, when a lens having a large numerical aperture (hereinafter, it is referred to as a “high NA”) is designed, a single lens structure is effective to solve problems such as an increase in process number during assembly, deterioration in production efficiency, and an increase in cost.
Furthermore, in the high NA objective lens, it is important to prevent deterioration in aberration. Therefore, it is important to satisfactorily correct various aberrations by providing an aspheric surface or the like.
Generally, an objective lens for use in recording/reproducing an optical recording medium has a peculiar shape, that is, a convex surface which has a large curvature and is directed to a light source. In particular if an objective lens is formed to have a high NA, a shape of the lens has great influence on not only a spherical aberration but also various aberrations.
Hence, in JP 2003-5032 A (corresponding to U.S. Pat. No. 7,110,344), a conditional expression relating to a sag amount is defined, and a value determined by the conditional expression is set in a predetermined allowable range, thereby preventing deterioration in various aberrations of the high NA lens.
Furthermore, the objective lens for recording/reproducing an optical recording medium tends to be too large in on-axis. Accordingly, in a high NA single lens, an on-axis thickness d is set to be in a predetermined range which is defined based on a relation between the thickness d and a focal length f, and so excellent image height characteristic are obtained (see JP 2001-324673 A (corresponding to U.S. Pat. No. 6,411,442) and JP 2003-5032 A).
However, in the objective lens described in JP 2003-5032 A, if the NA is increased to be in the range from about 0.7 to about 1.0 within the predetermined range defined by the conditional expression about the sag amount, it is not always possible to satisfactorily correct a spherical aberration.
Furthermore, in the objective lens described in JP 2001-324673 A and JP 2003-5032 A, if the NA is increased to the range from about 0.7 to about 1.0 within the predetermined range defined by the conditional expression of d/f, it is not always possible to satisfactorily correct high-order various aberrations with an image height characteristic being assessed with a wavefront aberration. In addition, in terms of a decrease in power for driving an objective lens and a decrease in size of apparatus, which is expected to be required in future, values relating to reducing weight of the objective lens and ensuring an operating distance are not always satisfactory ones.